The present invention relates to a technology of reducing cross talk in a semiconductor integrated circuit and a technology effectively applicable to a signal-processing LSI (or a large-scale semiconductor integrated circuit) for receiving and processing signals in a plurality of different frequency bands. More particularly, the present invention relates to a technology effectively applicable to a radio-communication LSI employed typically in a hand phone for processing a signal received by adoption of a super-heterodyne technique.
As a radio-communication system adopted in a hand phone, there is known a radio-communication system embracing the super-heterodyne technique as shown in FIG. 11. In the radio-communication system shown in FIG. 11, reference numeral 100 denotes an antenna for receiving a signal wave and reference numeral 101 denotes a reception/transmission changeover switch. Reference numeral 110 denotes a reception-system circuit for amplifying the signal received by the antenna 100 and demodulating the amplified signal. Reference numeral 120 denotes a transmission-system circuit for modulating a signal to be transmitted through the antenna 100 and converting the frequency of the signal. Reference numeral 130 is an oscillation-system circuit for generating a local oscillation signal required by the reception-system circuit 110 and the transmission-system circuit 120. Reference numeral 140 denotes a base-band-signal-processing circuit for carried out processing such as extraction of audio data from a signal received by the antenna 100 and conversion of the audio data into a train of voltage pulses. Reference numeral 150 is a system controller including a microcomputer for totally controlling the entire radio-communication system. The transmission/reception changeover switch 101 is controlled by a control signal TX/RX output by the system controller 150 to switch the mode of the radio-communication system from reception to transmission and vice versa.
The reception-system circuit 110 comprises a band-limiting filter (FLT) 111, a low-noise amplification circuit (LNA) 112, a down-conversion mixer (MIX) 113, a band-pass filter (BPF) 114, a programmable-gain amplifier (PGA) 115 and a demodulator (DeMOD) 116. The FLT 111 is typically an SAW filter for removing unnecessary waves from a signal received by the antenna 100. The LNA 112 is an amplifier for amplifying a signal passing through the band-limiting filter (FLT) 111. The MIX 113 is a converter for down-converting the frequency of the signal amplified by the amplification circuit (LNA) 112 into an intermediate frequency by mixing the signal with the local oscillation signal generated by the oscillation-system circuit 130. The BPF 114 is a filter for passing through a signal having the frequency corresponding to a difference in frequency between the signal amplified by the amplification circuit (LNA) 112 and the local oscillation signal. The programmable-gain amplifier (PGA) 115 is an amplifier capable of controlling a gain at which a signal output by the band-pass filter (BPF) 114 is amplified thereby to a desired level. The DeMOD 116 is a demodulator for modulating the signal with the amplitude thereof adjusted by the programmable-gain amplifier (PGA) 115 to a desired level into a base-band signal (I/Q).
The transmission-system circuit 120 comprises a modulator (MOD) 121, an up-conversion mixer (U-MIX) 122 and a power amplifier (PA) 123. The MOD 121 is a modulator for modulating a signal to be transmitted into an RF (radio frequency) signal. The signal to be transmitted is received from the base-band-signal-processing circuit 140 as a base-band signal (I/Q). The mixer (U-MIX) 122 is a converter for up-converting the frequency of the signal obtained as a result of modulation by the modulator (MOD) 121 into a desired transmission frequency by mixing the modulated signal with the local oscillation signal generated by the oscillation-system circuit 130. The PA 123 is an amplifier for amplifying the power of the signal to be transmitted after the frequency conversion prior to a transmission by way of the antenna 100.
The oscillation-system circuit 130 comprises a radio-frequency voltage-controlled oscillation circuit (RFVCO) 132, an intermediate-frequency voltage-controlled oscillation circuit (IFVCO) 131, a synthesizer (SYN) 133 and a buffer (BFF) 134. The RFVCO 132 is a voltage-controlled oscillation circuit for generating an RF (radio frequency) signal used in the down-conversion mixer (MIX) 113 and the up-conversion mixer (U-MIX) 122. On the other hand, the intermediate-frequency voltage-controlled oscillation circuit (IFVCO) 131 is a voltage-controlled oscillation circuit for generating an IF (intermediate frequency) signal, that is, a signal with a fixed frequency, required by the demodulator (DeMOD) 116 and the modulator (MOD) 121. The SYN 133 is a synthesizer for generating control voltages applied to the radio-frequency voltage-controlled oscillation circuit (RFVCO) 132 and the intermediate-frequency voltage-controlled oscillation circuit (IFVCO) 131 respectively. The control voltage applied to each of the VCO 131 and VCO 132 is generated in accordance with a difference in phase between a feedback signal generated by the VCO 131 and VCO 13 and a reference signal TCXO generated by an oscillation circuit employing a crystal oscillator exhibiting characteristics of high frequency precision and little temperature dependence. The difference in phase is obtained as a result of comparing the feedback signal with the reference signal TCXO. The BFF 134 is a buffer for supplying an oscillation signal generated by the RFVCO 132 to the down-conversion mixer (MIX) 113 employed in the reception-system circuit 110 and the up-conversion mixer (U-MIX) 122 employed in the transmission-system circuit 120 by proper distribution. It should be noted that the synthesizer (SYN) 133 and the radio-frequency voltage-controlled oscillation circuit (RFVCO) 132 constitute a closed-loop circuit known as a PLL (Phase Locked Loop) circuit. Similarly, the synthesizer (SYN) 133 and the intermediate-frequency voltage-controlled oscillation circuit (IFVCO) 131 constitute another closed-loop circuit also known as a PLL (Phase Locked Loop) circuit.
The radio-communication system shown in FIG. 11 comprises about 10 IC chips each implemented as a semiconductor integrated circuit. The IC chips are units of integration implementing the circuit blocks 112, 113, 115, 116 and so on. If the radio-communication system for processing transmitted and received signals comprises a plurality of IC chips, the number of components rises, inevitably increasing an area for mounting the components. For a portable electronic device such as a hand phone, however, a small size and low power consumption are mandatory requirements. Thus, reduction of the component count is a technological challenge of importance.
In order to reduce the number of components such as ICs composing the radio-communication system of a hand phone, inventors of the present invention developed an LSI allowing some of several circuit blocks shown in FIG. 11 to be integrated into a single semiconductor chip. FIG. 12 is a diagram showing a layout of circuit blocks initially considered to be put in the LSI. Circuit blocks of FIG. 12 identical with those shown in FIG. 11 are denoted by the same reference numerals as the latter. A comparison of FIG. 11 with FIG. 12 clearly indicates that circuit blocks shown in FIG. 12 are merely laid out along flows of received and transmitted signals as is the case with those shown in FIG. 11.
By simply arranging the circuit blocks into a layout on a semiconductor chip as shown in FIG. 12, however, a result of an interference-wave test clearly indicated a deteriorating CN (component to noise) ratio. To put it concretely, the result of the test indicated that, when an interference wave with an interfering frequency was introduced at xe2x88x9226 dB to a desired signal input through the antenna at xe2x88x9299 dB, the CN ratio deteriorated, causing a bit error rate to exceed a desirable level.
In order to solve the problem described above, the inventors of the present invention studied causes of the deterioration of the CN ratio accompanying introduction of an interference wave. Results of the study are explained as follows.
FIG. 13 is a diagram showing a frequency distribution of an interference wave and a desired wave with a deteriorated CN ratio caused by introduction of the interference wave. In FIG. 13, notations fW and fB denote a desired wave (or a received wave) and an interference wave respectively whereas notation fRELO denotes the RF (radio frequency) local oscillation signal to be mixed with the received signal (or the desired signal) in the down-conversion mixer (MIX) 113 shown in FIG. 12. Notation fIFW denotes a desired wave obtained as a result of frequency down conversion by mixing the RF local oscillation signal with the received signal. Notation fIFLO is an IF (intermediate frequency) signal generated by the intermediate-frequency voltage-controlled (IFVCO) 131. The frequency of the signal fIFLO is an intermediate frequency of typically 540 MHz.
Assume that the frequency of the desired wave fW is 940 MHz and the frequency of the RF local oscillation signal fRFLO is 1,165 MHz. In this case, in the down-conversion mixer (MIX) 113 converts the desired wave fW into the signal fIFW with an intermediate frequency of 225 MHz (=1,165 MHz xe2x88x92940 MHz). In this state, when an interference wave fB with a frequency of 935 MHz is received, noise components fN1 and fN2 appear as shown in FIG. 13.
The band-pass filter (BPF) 114 is capable of removing the noise component fN1 but not the noise component fN2. This is because the noise component fN2 has all but the same frequency as the signal fIFW""s intermediate frequency of 225 MHz obtained as a result of the down conversion by the down-conversion mixer (MIX) 113. From this consideration, the deterioration of the CN ratio is thought to be caused by the noise component fN2. As shown in FIG. 12, the intermediate-frequency voltage-controlled oscillation circuit (IFVCO) 131 is put in the same LSI as the down-conversion mixer (MIX) 113. In this case, it is feared that a noise caused by cross talk propagates from the intermediate-frequency voltage-controlled oscillation circuit (IFVCO) 131 to the down-conversion mixer (MIX) 113 through a semiconductor substrate, causing the CN ratio to deteriorate.
In order to solve the problem described above, the inventors of the present invention conceive that a noise component is a spurious noise obtained as result of mixing the interference wave, the local oscillation signals and the intermediate-frequency signals or mixing higher harmonics of the interference wave, the local oscillation signals and the intermediate-frequency signals. That is, the frequency fN of a noise component can be expressed as follows:
fN=A*fRFLOxc2x1B*fIFLOxc2x1C*fB
where the symbols A, B and C are each an integer whereas the symbol * denotes a multiplication operator. If 1,165 MHz, 540 MHz and 935 MHz are substituted for the RF local oscillation frequency fRFLO, the IF local oscillation frequency fIFLO and the interference-wave frequency fB respectively whereas xe2x88x922, 3 and 1 are substituted for the integers A, B and C respectively in the above equation, the intermediate frequency fN of the noise component is found to be 225 MHz. That is, the inventors of the present invention came to a conclusion that a noise component is indeed an intermediate-frequency spurious noise obtained as result of a synthesis of the interference wave, the local oscillation signals and the intermediate-frequency signals or a synthesis of higher harmonics of the interference wave, the local oscillation signals and the intermediate-frequency signals. At a development stage, the inventors of the present invention thought that, by mounting an LSI comprising circuit blocks like the ones shown in FIG. 12 on an SOI (Silicon on Insulator) substrate, it would be possible to reduce noise caused by cross talk propagating through the substrate. The inventors of the present invention also discovered, however, that the spurious noise could not be sufficiently reduced by merely using an SOI substrate.
In addition, as an application of the LSI described above, a signal-processing circuit to be used in a radio communication system adopting a single super-heterodyne technique has been assumed. If a double super-heterodyne technique is adopted, however, the number of local oscillation signals used in down conversion rises, increasing the number of mixing combinations of the local oscillation signals and the desired signal (received signal) or mixing combinations of higher harmonics of the local oscillation signals and the desired signal. An increased number of such combinations may result in more spurious noises giving rise to a problem of concern that there are more frequent cases in which the frequency of a spurious noise happens to almost coincide with the frequency of a signal obtained as a result a down conversion of a received signal, causing the CN ratio to deteriorate. It should be noted that the double super-heterodyne technique is a method whereby an intermediate-frequency signal obtained as a result of a down conversion of a received signal is further subjected to another down conversion by using a second mixer to generate a signal with en even lower frequency.
It is thus an object of the present invention to provide a signal-processing semiconductor integrated circuit that is used in a radio-communication system for signal processing to convert the frequency of a received signal (or a desired signal) into a lower frequency by mixing the received signal with a local oscillation signal, and is capable of reducing a CN ratio""s deterioration caused by a spurious noise.
It is another object of the present invention to provide a semiconductor integrated circuit for radio communication capable of reducing deterioration of a CN ratio.
The above and other objects of the present invention as well as novel features thereof will become apparent from a careful study of this specification with reference to accompanying diagrams.
An outline of representatives of the present invention disclosed in this specification is described as follows.
In a semiconductor integrated circuit obtained as a result of formation of a first circuit block and a second circuit block on a semiconductor substrate, the first and second circuit blocks are created respectively in a first island area and a second island area on the surface of the semiconductor substrate wherein the first and second island areas are each enclosed by an insulating isolation band; a semiconductor area having a resistance lower than a base-substance area is created on the base-substance area, which is an area excluding a region for forming active elements on the first island area and excluding a region for forming active elements on the second island area; and the semiconductor area having a relatively low resistance is connected to a voltage terminal.
Since the insulating isolation band for electrically separating the first and second circuit blocks from each other functions as capacitors at high frequencies, the circuit blocks appear to be connected to each other by the capacitors. By virtue of the arrangement described above, however, the coupling capacitance between the first and second circuit blocks decreases since the coupling capacitors are connected to each other in series. Thus, the number of cross-talk components propagating from the first circuit block to the second circuit block can be reduced. As a result, bad effects of noises can also be decreased as well.
In addition, it is desirable to form a relatively-low-resistance semiconductor area for separating the first and second circuit blocks from each other in parallel to mutually interfacing boundaries on the first and second island areas in a region between the first and second island areas and to connect the relatively-low-resistance semiconductor area to a voltage terminal wherein the resistance of the relatively-low-resistance semiconductor area for separating the first and second circuit blocks is made lower than the resistance of the region between the first and second island areas. In this way, the electric potential of a semiconductor base substance between the first and second circuit blocks is fixedly firmed, making it difficult for a noise to propagate from the first circuit block to the second one.
Furthermore, the first circuit block includes an oscillation circuit, and a third island area enclosed by an insulating isolation band is created in a region between the first and second island areas. In the third island area, there is created a third circuit block, which is a collection of circuits pertaining to neither a category of circuits each considered to be most likely a noise generator nor a category of circuits each possibly malfunctioning due to propagation of a noise. In addition, a semiconductor area having a resistance lower than a base-substance area is created on a base-substance area, which is an area excluding a region for forming active elements on the third island area, and is connected to a voltage terminal. In this way, the third island area, in which the third circuit block is formed, executes functions similar to the relatively-low-resistance semiconductor area for separating the first and second block circuits from each other. As a result, the electric potential of a semiconductor base substance between the first and second circuit blocks is fixedly firmed, making it difficult for a noise to propagate from the first circuit block to the second one.
Moreover, the semiconductor substrate is an SOI substrate in which a semiconductor layer is created on a support substrate, being separated from the support substrate by an insulating layer. The island areas described above are formed on the semiconductor layer. It is preferable to create the insulating isolation band described above by having the band penetrate the semiconductor layer to the insulating layer. In this way, a semiconductor area including a circuit considered to be most likely a noise generator is cut off from a semiconductor area including a circuit easily hurt by a bad effect of a noise, being most likely led to a malfunction by the insulating isolation band. As a result, the quantity of noise and the number of noises propagating through the semiconductor base substance can be reduced.
In addition, the active elements described above are each a vertical-type bipolar transistor with the collector thereof implemented by a relatively-low-resistance embedded semiconductor region formed by embedding the region in the semiconductor layer. It is preferable to create the relatively-low-resistance embedded semiconductor region by using the same process as the semiconductor area as described above. As a result, a noise-proof semiconductor integrated circuit can be implemented without a need to newly add a process.
To put it concretely, a configuration of a signal-processing semiconductor integrated circuit includes:
a first oscillation circuit for generating a first oscillation signal;
a second oscillation circuit for generating a second oscillation signal;
an oscillation control circuit for generating control voltages applied to the first and second oscillation circuits respectively;
a first mixer circuit for converting the frequency of a signal received by an antenna by mixing the received signal with the first oscillation signal;
an amplification circuit for amplifying a signal with a frequency obtained as a result of frequency conversion carried out by the first mixer circuit;
a demodulation circuit for demodulating a signal obtained as a result of amplification carried out by the amplification circuit; and
a second mixer circuit for converting the frequency of a signal to be transmitted by the antenna by mixing the signal to be transmitted with the second oscillation signal,
wherein at least a first group of circuits and a second group of circuits are mounted on a semiconductor substrate by separating them from each other where the first group of circuits comprises the first mixer circuit and the first oscillation circuit whereas the second group of circuits comprises the second oscillation circuit, the amplification circuit and the demodulation circuit. As a result, it is possible to prevent the CN ratio in the first mixer from deteriorating due to a spurious noise generated by the second oscillation circuit.
If there are further provided a modulation circuit for generating the to-be-transmitted signal to be mixed by the second mixer with the second oscillation signal, and a control circuit for controlling internal components of the signal-processing semiconductor integrated circuit, in addition to the separation of the first group of circuits comprising the first mixer circuit and the first oscillation circuit from the second group of circuits comprising the second oscillation circuit, the amplification circuit and the demodulation circuit, it is desirable to place one of the second mixer circuit, the oscillation control circuit, the modulation circuit and the control circuit or any combination of the second mixer circuit, the oscillation control circuit, the modulation circuit and the control circuit between the first group of circuits and the second group of circuits. In this way, it is possible reduce the amount of wasted space and to reduce the deterioration of the CN caused by a spurious noise.
If there is further provided a third mixer circuit for converting the frequency of a signal generated as a result of the frequency conversion carried out by the first mixer circuit by mixing the signal with the second oscillation signal generated by the second oscillation circuit in a second-stage frequency conversion, it is preferable to separate the first group of circuits comprising the first mixer circuit and the first oscillation circuit from a third group of circuits comprising the second oscillation circuit, the amplification circuit, the demodulation circuit and the third mixer circuit. In this case, it is also nice to place one of the second mixer circuit, the oscillation control circuit, the modulation circuit and the control circuit or any combination of the second mixer circuit, the oscillation control circuit, the modulation circuit and the control circuit between the first group of circuits and the third group of circuits. In this way, even for a signal-processing LSI employed in a radio-communication system adopting the double super-heterodyne technique, it is possible to reduce the amount of wasted space on the semiconductor substrate and to reduce the deterioration of the CN caused by a spurious noise.